Fluoxetine destabilizes home cage activity. (A) Representative data showing nocturnal locomotor activity levels in home cage (left) and frequency facilitation at the mossy fiber synapse induced by 1 Hz stimulation (right) in a control (CNT) mouse. (B) Averaged data of nocturnal activity levels in control mice. (C) Representative nocturnal activity and synaptic facilitation, and (D) averaged nocturnal activity in mice treated with 14 mg/kg/day fluoxetine (FLX14). (E) Representative nocturnal activity and synaptic facilitation, and (F) averaged nocturnal activity in mice treated with 22 mg/kg/day fluoxetine (FLX22). Representative behavioral and electrophysiological data were obtained from the same mice. Averaged data are presented as mean ± s.e.m. (G) Coefficient of variation (CV) of activity levels during last 2 weeks of treatments. FLX22 (n = 29) mice showed larger variation than control (n = 27, rank sum difference = 29.48, p < 0.0001) and FLX14 (n = 9, rank sum difference = 27.11, p < 0.0001). There was no significant difference between control and FLX14 (rank sum difference = 2.37, p > 0.05). (H) CV of activity was negatively correlated with the magnitude of frequency facilitation at the mossy fiber synapse in FLX22 mice (n = 16, p = 0.0007, r2 = 0.569). Each symbol represents a single mouse.